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Only a fraction of the total power emitted by the sun impinges on an object in space which is some distance from the sun. The solar irradiance (H0 in W/m2) is the power density incident on an object due to illumination from the sun. At the sun's surface, the power density is that of a blackbody at about 6000K and the total power from the sun is this value multiplied by the sun's surface area. However, at some distance from the sun, the total power from the sun is now spread out over a much larger surface area and therefore the solar irradiance on an object in space decreases as the object moves further away from the sun.

The solar irradiance on an object some distance D from the sun is found by dividing the total power emitted from the sun by the surface area over which the sunlight falls. The total solar radiation emitted by the sun is given by σT4 multiplied by the surface area of the sun (4πR2sun) where Rsun is the radius of the sun. The surface area over which the power from the sun falls will be 4πD2. Where D is the distance of the object from the sun. Therefore, the solar radiation intensity, H0 in (W/m2), incident on an object is:

where:
Hsun is the power density at the sun's surface (in W/m2) as determined by Stefan-Boltzmann's blackbody equation;
Rsun is the radius of the sun in meters as shown in the figure below; and
D is the distance from the sun in meters as shown in the figure below.

At a distance, D, from the sun the same amount of power is spread over a much wider area so the solar radiation power intensity is reduced.

Distance from the Sun - Radiation Intensity Calculator

The table below gives standardised values for the radiation at each of the planets but by entering the distance you can obtain an approximation.

Planet Distance (x 109 m) Mean Solar Irradiance (W/m2)
Mercury 57 9116.4
Venus 108 2611.0
Earth 150 1366.1
Mars 227 588.6
Jupiter 778 50.5
Saturn 1426 15.04
Uranus 2868 3.72
Neptune 4497 1.51
Pluto 5806 0.878